Extraction of precious minerals such as gold from an ore or concentrate using cyanide leaching is well-known. The cyanide leaching must be conducted under alkaline conditions.
Gold in sulphide mineralisation can occur in several forms:
                free gold and electrum and fine inclusions of these particles in sulphide minerals        gold compounds (tellurides and selenides)        gold locked in the lattice of pyrite, arsenopyrite, stibnite etc (invisible gold)        
In free milling ores, particulate free gold and electrum can be recovered by conventional gravity and cyanidation methods. When these particles are present as fine inclusions in sulphide minerals, fine grinding is used to liberate the particles prior to cyanide leaching.
Fine gold particles locked in other minerals can be liberated by fine grinding from p80≈70μ to p80≈12μ. This liberation of fine gold particles from sulphides and quartz gange particles by fine grinding and cyanide leaching is well known.
It has been found that fine grinding to circa 10 microns and leaching with sodium cyanide can recover the majority of the free gold and the gold present as gold compounds. High pH (˜11-12) high cyanide concentration (2-5000 ppm NaCN vs 200-300 ppm used conventionally) and long leach times are required, (48-72 hours vs 18-24 hours required conventionally). Under these conditions, recoveries of 80-90% for gold can be achieved with consumptions of lime of 5-15 kg/t and 12-20 kg/t of sodium cyanide. Normal commercial operation results in consumption of 1.5-5 kg/t of lime and 1.5-2.5 kg/t of sodium cyanide.
During the fine grinding process, the sulphide minerals are also finely ground and a large surface area of fresh unoxidised sulphide mineralisation is exposed. It is this sulphide surface which reacts with cyanide during cyanide leaching for gold extraction to form thiocyanates and other thio species. This results in the high cyanide consumption observed during cyanide leaching of finely ground sulphidic gold ores.
It has been reported that fine grinding of pyrite concentrates by stirred ball mills, the LURGI centrifugal ball mill and the Sweco vibrating mill produced significant recoveries in gold extraction by cyanide leaching. The grind sizes achieved were p50 of 2-8μ. During leaching, the cyanide consumption was double that observed at a coarser grind.
Some types of ores are not able to be leached using cyanide, as the precious minerals are locked in the ore in such a manner that extraction using cyanide does not work. These types of ores can be called refractory ores. A typical refractory ore comprises a sulphide ore and a carbonaceous ore.
In order to release the precious minerals from refractory ores (thereby allowing cyanide leaching to be carried out) it is known to initially pre-treat the ore by roasting, by bacterial leaching, and to use chemical leaching at elevated temperatures and pressures all of which increases the cost of recovering the precious minerals from the ore/concentrates.
Most pre-treating leaching processes use oxygen and acidic conditions. Once the ore/concentrate has been treated, the acid must be neutralised prior to cyanide leaching which requires less acid or more alkaline conditions. This increases the cost of extraction of the precious minerals.
Alkaline leaching is known but alkaline leaching is not very efficient with refractory materials.
International patent application PCT/AU99/00795 describes an alkaline leaching process to extract precious metals such as gold. The alkaline leaching process requires fine grinding of the ore and lime and/or limestone is used as the alkaline reagent. Oxygen is used as the oxidising agent and the process is continued until approximately 90% of sulphide oxidation had taken place. The resultant product is subjected to cyanide leaching to remove gold and other precious minerals.
A disadvantage with the above alkaline leaching process is that the amount of cyanide consumed during the extraction process is rather high which adds to the cost of the overall process. Also, the reaction time is quite long.
During the fine grinding process, the sulphide minerals are finely ground and a large surface area of fresh unoxidised sulphide mineralisation is exposed. It is this sulphide surface which reacts with cyanide during cyanide leaching for gold extraction to form thiocyanates and other thio species. This results in the high cyanide consumption observed during cyanide leaching of finely ground sulphidic gold ores. In our previous process described in the above International patent application, the sulphide was almost totally oxidised but even so, there was still a large consumption of cyanide.